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diy solar

12V to 24V Conversion Question

You know - at the power levels I was listing above such as 22kwh/day.... a vehicle alternator is just not in the right scale of ability for me. I've tried to think about using the car engine - but it just doesn't help enough at 40a. I wish there were 3000w-5000w generator add-ons for engines instead of trying to fool around with 5000w/12v = 417a alternators. I could use 5000w of charging during a 4hr daily driving trip or even parked at a site and run the engine as vehicles are usually pretty quiet. If the generator output 120v/240v one could plug it in (or hook it to) the external grid input a lot of trailers have.

EV trucks are starting to offer this level of external power. @Will did an EV truck recently that could output 7000w.
 
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OffGridInTheCity said:
You know - at the power levels I was listing above such as 22kwh/day.... a vehicle alternator is just not in the right scale of ability for me. I've tried to think about using the car engine - but it just doesn't help enough at 40a. I wish there were 3000w-5000w generator add-ons for engines instead of trying to fool around with 5000w/12v = 417a alternators. I could use 5000w of charging during a 4hr daily driving trip or even parked at a site and run the engine as vehicles are usually pretty quiet. If the generator output 120v/240v one could plug it in (or hook it to) the external grid input a lot of trailers have.

EV trucks are starting to offer this level of external power. @Will did an EV truck recently that could output 7000w.
Click to expand...

XCITE PTO SHAFT DRIVEN GENERATOR​


A PTO was an option when I ordered my F-350, but I didn't add it. It's a lot less expensive to add solar to the RV.
 
@HRTKD I've decided to follow your lead on going 12V instead of 24V. I'm thinking I will go with 2p4s as that seems to be preferred over 4s2p, to keep the cells evenly charged with the added benefit of using one BMS rather than two. I am curious how you chose to connect the cells; did you use rigid bars that came with the batteries or did you go with cable between the cells (and what gauge if you did)?
 
riverbug said:
@HRTKD I've decided to follow your lead on going 12V instead of 24V. I'm thinking I will go with 2p4s as that seems to be preferred over 4s2p, to keep the cells evenly charged with the added benefit of using one BMS rather than two. I am curious how you chose to connect the cells; did you use rigid bars that came with the batteries or did you go with cable between the cells (and what gauge if you did)?
Click to expand...

I think your observation about the preference of 2p4s over 4s2p is wrong. There is a vocal minority (but they're nice guys) that recommend 2p4s. I disagree with them. The 4s2p has the following benefits:
1. Finer granularity of monitoring. One of the parallel cells could be bad and you might never know it.
2. Better balancing. The passive balancing is more likely to be successful with the smaller amps.
3. Redundancy. Should you need to take one battery offline, the other battery can continue.
4. Greater amps. With two BMS you have twice the amps.
5. Easier to move. Half the weight!

Yes, 4s2p costs more due to the second BMS. But I like the benefits.
 
Perhaps we should clarify here a bit.....
4s2p (4 in series of LifePo4 = 12v nominal) and 2p (2 in parallel) expresses paralleling the cells. 1 BMS is all that's needed.
2p4s is the same. 2 in parallel, 4 in series. The p and s are reversed, but the meaning is the same. 1 BMS is all that's needed.
*There is no difference by writing 4s2p or 2p4s - both are 12v battery banks with only 1 BMS needed.

Was the idea that 4s1p (12v) times 2 of these in series = 24v? In which case I would agree that 2 independent BMSs (for each 4s) would be needed and the 2 x 12v batteries would not stay in balance over the long haul. But one could do 8s1p = 24v and again have 1 BMS.
 
HRTKD said:
I think your observation about the preference of 2p4s over 4s2p is wrong. There is a vocal minority (but they're nice guys) that recommend 2p4s. I disagree with them. The 4s2p has the following benefits:
1. Finer granularity of monitoring. One of the parallel cells could be bad and you might never know it.
2. Better balancing. The passive balancing is more likely to be successful with the smaller amps.
3. Redundancy. Should you need to take one battery offline, the other battery can continue.
4. Greater amps. With two BMS you have twice the amps.
5. Easier to move. Half the weight!

Yes, 4s2p costs more due to the second BMS. But I like the benefits.
Click to expand...
Oh, interesting. I don't mind spending an extra $100 on a BMS if that's a better solution. Sometimes navigating information is hard; hard to know what is definitively accurate, conjecture, or just wrong. Once I got down the rabbit hole of benefits of 2p4s I guess everything seemed to point to that being the best solution and it seemed each additional video or forum post continued to reinforce that. Early yesterday I ordered the second BMS, then cancelled it early this morning. I did go back and forth.

Since I certainly don't know better, I'm going to follow your lead again and go with 4s2p - I'll have to reorder that BMS. :)

Would you mind a quick comment on rigid vs cable connectors for cells in an RV (is vibration a concern with rigid)?

Tell me if you would prefer I just post questions without hailing you directly...but I really do appreciate the help.
 
OffGridInTheCity said:
Perhaps we should clarify here a bit.....
4s2p (4 in series of LifePo4 = 12v nominal) and 2p (2 in parallel) expresses paralleling the cells. 1 BMS is all that's needed.
2p4s is the same. 2 in parallel, 4 in series. The p and s are reversed, but the meaning is the same. 1 BMS is all that's needed.
*There is no difference by writing 4s2p or 2p4s - both are 12v battery banks with only 1 BMS needed.

Was the idea that 4s1p (12v) times 2 of these in series = 24v? In which case I would agree that 2 independent BMSs (for each 4s) would be needed and the 2 x 12v batteries would not stay in balance over the long haul. But one could do 8s1p = 24v and again have 1 BMS.
Click to expand...
Well, perhaps that's my misunderstanding of using those acronyms. My reference to 2p4s was meant as 8 cells to create one 12V battery where 4 pair of cells are connected in parallel and then the 4 parallel pairs are joined in series, and using one BMS (4s).

4s2p to me meant two separate 12V batteries each with it's own BMS and where the 4 cells in each battery are connected in series, and the two batteries are connected in parallel.

Wrong apparently?
 
riverbug said:
Well, perhaps that's my misunderstanding of using those acronyms. My reference to 2p4s was meant as 8 cells to create one 12V battery where 4 pair of cells are connected in parallel and then the 4 parallel pairs are joined in series, and using one BMS (4s).

4s2p to me meant two separate 12V batteries each with it's own BMS and where the 4 cells in each battery are connected in series, and the two batteries are connected in parallel.
Click to expand...
Yea, 4s2p is still 1 battery. 2 separate 4s1p batteries connected in series or parallel would still be 2 separate batteries and need 2 BMSs and because the BMSs are separate, there's nothing to keep the cells in the batteries balanced to each other.

You can do 8s1p = 24v as a single battery and use an 8s BMS and this will keep all 8 cells in balance to each other. Not promoting DALY BMS but just to show you that 8s is in the mainstream of availability - https://www.amazon.com/DALY-LiFePO4-Battery-Management-Protection/dp/B087BXR4TZ/ref=sr_1_3

Or do 4s2p = 12v as a single battery and have 1 BMS.
 
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riverbug said:
Would you mind a quick comment on rigid vs cable connectors for cells in an RV (is vibration a concern with rigid)?
Click to expand...

I've had no problems with the rigid bus bars connecting my cells. My batteries are deployed in my toy hauler trailer. The trailer has seen some very rough roads. Rough enough that I've broken things on the bottom of the trailer like the black tank valve. I check the connections in my system once a year. Only one of the cells could be tightened and it was just a smidge. Do use a torque wrench. The bolts holding down the bus bars on my cells use no lock washers or thread locker. There are whole threads that address how to keep the cell bolts/nuts tight.

The critical point to know about DIY LiFePO4 batteries, especially those deployed in a mobile application, is that the cells need to be in a compression fixture. This keeps them from moving against each other and it also prevents the cells from bloating. A very thin layer of nonconductive material between the cells is recommended. Flexible cutting board material is ideal for this. There are also (very lengthy) threads that cover this topic. The most talked about item in those threads is how tight to make the compression. Too tight is bad, too loose is useless. Good and snug is about right.

riverbug said:
Tell me if you would prefer I just post questions without hailing you directly...but I really do appreciate the help.
Click to expand...

I get notified either way.
 
riverbug said:
Well, perhaps that's my misunderstanding of using those acronyms. My reference to 2p4s was meant as 8 cells to create one 12V battery where 4 pair of cells are connected in parallel and then the 4 parallel pairs are joined in series, and using one BMS (4s).

4s2p to me meant two separate 12V batteries each with it's own BMS and where the 4 cells in each battery are connected in series, and the two batteries are connected in parallel.

Wrong apparently?
Click to expand...

You had it right.
 
I don't understand the application of using 1p though. How can I have one cell or battery in parallel with itself? Wouldn't that be an oxymoron?
 
riverbug said:
I don't understand the application of using 1p though. How can I have one cell or battery in parallel with itself? Wouldn't that be an oxymoron?
Click to expand...
We've been discussing this as if you have 8 x LifePo4 cells that you want to hookup up. If you had 8 individual cells - you could hook up all 8 in series for 8s1p. or 2 sets of 4s1p of 1 set of 2s2p. But perhaps you don't have individual LifePo4 cells - e.g. the 'green cellls' in the pics above.

What do you have exactly, maybe a pic ?
 
OffGridInTheCity said:
No, it means 1 battery where each s has 2 cells in parallel as in the pic above. For example, 16s10p = 1 battery where each s has 10cells in parallel.
Click to expand...

Your picture is wrong. 2p4s is one battery using four sets of two cells in parallel. 4s1p is a single 4s battery. 4s2p is two 4s batteries in parallel.

When we talk about PV strings, 4s2p is two sets of four panels in series, then the two strings are in parallel.
 
HRTKD said:
Your picture is wrong. 2p4s is one battery using four sets of two cells in parallel. 4s1p is a single 4s battery. 4s2p is two 4s batteries in parallel.
Click to expand...
Can only lead to water....

HRTKD said:
When we talk about PV strings, 4s2p is two sets of four panels in series, then the two strings are in parallel.
Click to expand...
Panels are batteries - no BMS needed so it doesn't matter. Different topics.
 
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OffGridInTheCity said:
Perhaps we should clarify here a bit.....
4s2p (4 in series of LifePo4 = 12v nominal) and 2p (2 in parallel) expresses paralleling the cells. 1 BMS is all that's needed.
2p4s is the same. 2 in parallel, 4 in series. The p and s are reversed, but the meaning is the same. 1 BMS is all that's needed.
*There is no difference by writing 4s2p or 2p4s - both are 12v battery banks with only 1 BMS needed.

Was the idea that 4s1p (12v) times 2 of these in series = 24v? In which case I would agree that 2 independent BMSs (for each 4s) would be needed and the 2 x 12v batteries would not stay in balance over the long haul. But one could do 8s1p = 24v and again have 1 BMS.
Click to expand...
Actually 4s2p is two BMS's. One for each 4s.
Two batteries in parallel.
 
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